1. Field of the Invention
The present invention relates to spinal implants, and, more particularly, to intervertebral disc prostheses.
2. Description of Related Art
The spinal column comprises a series of vertebrae stacked on top of each other. There are typically seven cervical (neck), twelve thoracic (chest), and five lumbar (low back) segments. Each vertebra has a cylindrical shaped vertebral body in the anterior portion of the spine with an arch of bone to the posterior, which covers the neural structures. Each vertebral body includes superior and inferior endplates, which are respectively surrounded by superior and inferior bony rings, called ring apophyses. Between each vertebral body is an intervertebral disc, a cartilaginous cushion to help absorb impact and dampen compressive forces on the spine. To the posterior, the laminar arch covers and protects the neural structures of the spinal cord. At the junction of the arch and anterior vertebral body are articulations to allow movement of the spine.
Various types of problems can affect the structure and function of the spinal column. These can be based on degenerative conditions of the intervertebral disc or the articulating joints, traumatic disruption of the disc, bone or ligaments supporting the spine, tumor or infection. In addition, congenital or acquired deformities can cause abnormal angulation or slippage of the spine. Slippage (spondylolisthesis) anterior of one vertebral body on another can cause compression of the spinal cord or nerves. Patients who suffer from one of more of these conditions often experience extreme and debilitating pain, and can sustain permanent neurological damage if the conditions are not treated appropriately.
One treatment for spinal diseases and injuries is the removal and replacement of the intervertebral disc with a prosthetic device. Some intervertebral prosthetic devices provide a degree of pivotal and rotational movement, while others promote fusion of adjacent vertebrae. Typical non-fusion prosthetic discs, that provide a degree of pivotal and rotational movement, have rigid attachment members for attaching to adjacent vertebrae. The space between the attachment members is usually occupied by a core that generally includes either one or a plurality of elements that move relative to the fixation elements and/or each other. The elements of the core can be formed from polymers, ceramic materials, metals and combinations thereof. The core can also be formed as a single elastomeric element that provides relative motion between the attachment elements due to its material deformation. However, an elastomeric core may not match the kinetics of a natural disc and can eventually exhibit signs of fatigue. Some artificial disc cores have been proposed that include mechanical elements or mechanisms such as dashpots, springs, gears, dovetails, hinges, cams and bar linkages. Such prosthetic discs may require complicated assembly steps to assemble the attachment members and the core, due to the assembly of a large number of parts, and may tend to wear out over time as various mechanical elements fail. Further, conventional prosthetic discs may not replicate the quality or range of natural spinal movement to an acceptable degree. It would be desirable to provide an intervertebral disc prosthesis having a minimum number of separate components, which tends to resist component wear, and which replicates natural spinal movements as closely as possible.